Mega cisterna magna, arachnoid cyst, and Dandy Walker variant | First Look MRI

Midline posterior fossa fluid collections in adults usually represent benign congenital enlargement of the the cisterna magna. Arachnoid cysts of the posterior fossa are also possible and may be indistinguishable by imaging. Think of arachnoid cyst if the fluid is asymmetric and produces significant mass effect. Mega cisterna magna, arachnoid cyst, and Dandy Walker variant